1. Field of the Invention
The present invention relates to a temperature compensating circuit for correcting an output voltage of a temperature sensor circuit with a temperature.
2. Description of the Related Art
In general, an output of a sensor fluctuates due to a temperature. For example, in a pressure sensor, a zero-point offset voltage of the sensor output signal nonlinearly changes with respect to the temperature. This phenomenon will be described with reference to FIG. 9. In the sensor output, the zero-point offset voltage should be held constant regardless of a temperature Ta, as indicated by a curve 61. However, in fact, the zero-point offset voltage varies to a positive or negative side without regularity as indicated by curves 62 to 64. For those sensors, it is necessary that the individual sensors are subjected to different temperature corrections, respectively, and to achieve this, there is required a circuit capable of freely changing a positive correction, a negative correction, and their correction quantity. In order to solve the above problem, for example, a method disclosed in JP 06-174489 A has been proposed. That is, in a circuit diagram of FIG. 5, reference numeral 1 denotes a temperature compensating circuit. A power supply Vcc and an adjusted voltage Vi that is divided by resistors R5 and R6 are supplied to terminals 11 and 12 of the temperature compensating circuit 1. The adjusted voltage Vi is supplied to positive terminals of two operational amplifiers OP1 and OP2. The respective operational amplifiers OP1 and OP2 constitute a V/I converter (voltage/current converter) together with transistors Tr1 and Tr2. Currents Ia and Ib that are in proportion to the adjusted voltage Vi are supplied to temperature compensation resistors R1 and R2 which are different in temperature coefficients TC1 and TC2, respectively, by means of the V/I converter. Those currents Ia and Ib flow in transistors Tr3 and Tr4. The transistors Tr3 and Tr5, Tr4 and Tr6, and Tr7 and Tr8 constitute current mirror circuits, respectively. A current that is equal to a current that flows in the temperature compensation resistors R1 and R2 is allowed to flow in the transistors Tr5 and Tr6 as well as Tr7 and Tr8. Voltage divider resistors R3 and R4 are connected between the power supply Vcc and the ground G. The transistor Tr5 is connected in parallel to the resistor R3 that is connected at the power supply Vcc side, and the transistor Tr7 is connected in parallel to the resistor R4 that is connected at the ground G side. The current Ia of the transistor Tr5 flows into a connection point 14 between the voltage divider resistors R3 and R4, and the current Ib of the transistor Tr7 flows out of the connection point 14. The voltage at the connection point 14 is derived from a terminal 13 as an output Vo of the temperature compensating circuit 1. The output Vo of the temperature compensating circuit 1 is connected to a negative input terminal of the operational amplifier OP3 which is connected with the sensor output, and the sensor output that varies due to the temperature is subjected to temperature compensation. The provision of plural temperature compensating circuits enables the sensor output voltage that nonlinearly changes with respect to the temperature to be corrected. This structure will be described with reference to FIG. 7. Four temperature compensating circuits 1 to 4 are prepared. The respective temperature compensating circuits 1 to 4 set the resistances of the internal temperature compensation resistors R1 and R2, individually. The first temperature compensating circuit 1 satisfies TC1>TC2 so as to conduct a negative temperature correction when the temperature is lower than a set temperature Ta=25° C., the second temperature compensating circuit 2 satisfies TC1<TC2 so as to conduct a positive temperature correction when the temperature is lower than the set temperature. The third temperature compensating circuit 3 satisfies TC1>TC2 so as to conduct a negative temperature correction when the temperature is equal to or higher than the set temperature, and the fourth temperature compensating circuit 4 satisfies TC1<TC2 so as to conduct a positive temperature correction when the temperature is equal to or higher than the set temperature. Also, the respective temperature compensating circuits 1 to 4 are supplied with adjusted voltages, and can freely set the correction quantities, individually. Then, in the respective temperature compensating circuits 1 to 4, the voltage divider resistors R3 and R4 in FIG. 5 are omitted, and shared voltage divider resistors Ra and Rb are disposed instead of the voltage divider resistors R3 and R4. Accordingly, the connection point between the transistor Tr5 and the transistor Tr7 is derived from the terminal 13, and is connected to the shared voltage divider resistors Ra and Rb through contact points 6 and 7 which will be described later.
A temperature detecting circuit 5 is disposed in addition to the respective temperature compensating circuits 1 to 4, and the temperature detecting circuit 5 turns on/off the respective contact points 6 and 7 based on the detected temperature. When the temperature Ta is equal or higher than the set temperature 25° C., the temperature detecting circuit 5 turns off the contact point 6, and turns on the contact point 7. When the temperature Ta is lower than the set temperature, the temperature detecting circuit 5 turns on the contact point 6, and turns off the contact point 7. Then, the outputs of the temperature compensating circuits 1 and 2 which are used when the temperature Ta is lower than the set temperature are connected to the connection point 15 of the voltage divider resistors Ra and Rb through the contact point 6. The outputs of the temperature compensating circuits 3 and 4 which are used when the temperature Ta is equal to or higher than the set temperature are connected to the connection point 15 through the contact point 7. Then, the output Vo of the connection point 15 of the voltage divider resistors Ra and Rb is input to a negative terminal of the operational amplifier OP3 which is connected with the sensor output, to compensate the sensor output that fluctuates due to the temperature with a temperature. For example, when it is assumed that the sensor output characteristic that corrects the temperature is indicated by a curve 62 of FIG. 9, it is necessary to conduct the negative temperature correction when the temperature Ta is equal to or higher than the set temperature 25° C., and to conduct the positive temperature correction when the temperature Ta is lower than the set temperature 25° C. Also, it is necessary to set the correction quantity to be relatively larger when the temperature Ta is equal to or higher than the set temperature, and to set the correction quantity to be relatively smaller when the temperature Ta is lower than the set temperature. For that reason, the temperature compensating circuits 1 and 4 are selected. The remaining temperature compensating circuits 2 and 3 are prevented from outputting by cutting off a line that inputs the adjusted voltage Vi to the terminal 12. Also, the voltage divider resistors R5d and R6d are adjusted so that a large adjusted voltage Vi is input to the temperature compensating circuit 4 which conducts a large negative temperature correction. The voltage divider resistors R5a and R6a are adjusted so that a small adjusted voltage Vi is input to the temperature compensating circuit 1 which conducts a small positive temperature correction. With the above structure, the curves 52 and 54 in FIG. 8 are selected with the adjusted correction quantities. Then, the outputs of the respective temperature compensating circuits 1 and 4 are selected by the temperature detecting circuit 5 based on the temperature, and then input to the voltage divider resistors Ra and Rb. The correction quantity corresponding to the temperature is generated at the connection point 15 of the voltage divider resistors Ra and Rb through the principle described in a first embodiment of JP 06-174489 A. The output Vo of the temperature compensating circuit is input to the operational amplifier OP3 to conduct the temperature correction of the sensor output.
However, in the conventional method, there is a possibility that the temperature correction having a discontinuous characteristic is conducted before and after one temperature compensating circuit is changed over to another temperature compensating circuit. For example, as shown in FIG. 8, when the temperature compensating circuit changes over to another temperature compensating circuit at the set temperature 25° C., there is the continuity of the correction quantity of the temperature correction. However, when the temperature compensating circuit changes over to another temperature compensating circuit at another temperature, the continuity is lost.
Also, because four temperature compensating circuits are provided, the circuit scale becomes larger.